Rhabdoid tumor is a rapidly fatal malignancy that generally presents in the first two years of life. The most common sites of presentation are the brain and kidney. We and others used a combination of cytogenetic and molecular studies to define a critical region in 22q11.2 that contained a rhabdoid tumor suppressor gene. Recently, Versteege et al (1998) reported hSNF5/INI1 as a candidate gene for renal and extrarenal rhabdoid tumors. We identified germline and somatic mutations of INI1 in pediatric rhabdoid tumors of the central nervous system (Biegel et al, 1999), and have now documented germline mutations in patients with primary tumors of the brain and kidney. INI1 is a member of the SWIJSNF complex, one of several cell- cycle and ATP-dependent nucleosome remodeling complexes in mammalian cells. The SWTJSNF complex functions in both activation and repression of downstream target genes. In this proposal, we will define the spectrum of INI1 deletions and mutations in pediatric rhabdoid tumors of the brain, kidney and soft tissues. The frequency of inherited and de novo germline mutations in patients and their parents will be determined. Homozygous deletion of INI1 in chromosome band 22q 11.2 has been observed in 11 percent of tumors, whereas nonsense and frameshift mutations have been demonstrated in almost 50 percent of cases. In contrast, approximately 25 percent of primary tumors demonstrate alterations of the exon-intron 1 region of INI1 that result in absent INI1 gene expression. We propose that genomic rearrangements in this region or hypermethylation of CpG dinucleotides could decrease INI1 expression. Monosomy or deletion of the other chromosome 22 would thus result in homozygous inactivation of INI1, leading to tumor formation. The specificity of INTl mutations for rhabdoid tumors will also be addressed in this program, with particular emphasis on evaluating a larger series of medulloblastoma-primitive neuroectodermal tumors and choroid plexus carcinomas of the brain, as well as a series of pediatric rhabdomyosarcomas. These studies will lead to a better understanding of the role of INTl in the development of malignancy, improvements in determining diagnosis and prognosis for patients and their families, and ultimately, biologically based treatment strategies.